Stronger unions also have their issues. I strongly disagree that union-only shops should be allowed.
While I may be free to quit a union job, it feels quite gangster to say that I have to participate in one to take a job. I don't think the tradeoffs -- benefits from a union versus costs involved, especially when it's exclusively to pursue a career at a specific company -- favor unions.
That said, I do think unions could be stronger, and get a bad rap. To me, this seems like a problem the larger/stronger unions caused (and still cause - see police unions).
Your language - "be part of the group or don't work here" - is also a major disincentive. Artificially created black and white scenarios are reductionist.
Be part of the group or don't work here. My justification is that it drastically increases the strength of the union.